1. Field of the Invention
The present invention relates to the field of semiconductor processing. More particularly, the present invention relates to methods and apparatus for chemically mechanically polishing substrates with increased uniformity and reduced cost. The invention provides apparatus and methods to improve the uniformity of the rate at which material is removed from different locations on the substrate, and thereby increasing the number of useful die which are ultimately recovered from the substrate. Additionally, the present invention provides apparatus and methods for simultaneously polishing multiple substrates on a single polishing pad, thereby increasing the productivity of the chemical mechanical polishing apparatus.
2. Background of the Art
Chemical mechanical polishing, commonly referred to as CMP, is a method of planarizing or polishing substrates. CMP may be used as the final preparation step in the fabrication of substrates from semiconductor slices to provide substantially planar front and back sides thereon. CMP is also used to remove high elevation features, or other discontinuities, which are created on the outermost surface of the substrate during the fabrication of microelectronic circuitry on the substrate.
In a typical prior art CMP process, a large rotating polishing pad, which receives a chemically reactive slurry thereon, is used to polish the outermost surface of the substrate. To position the substrate on the polishing pad, the substrate is located in a carrier. The carrier is received on, or directly above, the polishing pad, and it maintains a bias force between the surface of the substrate and the rotating polishing pad. The carrier may also oscillate, vibrate or rotate the substrate on the polishing pad. The movement of the slurry whetted polishing pad across the planar face of the substrate causes material to be chemically mechanically polished from that face of the substrate.
One recurring problem with CMP processing is the tendency of the process to differentially polish the planar surface of the substrate, and thereby create localized over-polished and under-polished areas on the substrate. One area on the surface of a substrate where over-polishing commonly occurs is adjacent the substrate edge. When such edge over-polishing occurs, the polished substrate takes on a convex shape, i.e., it is thicker in the middle and thinner along its edge. If the substrate is to be further processed, such as by photolithography and etching, this thickness variation makes it extremely difficult to print high resolution lines on the substrate. If CMP is used to remove high elevation features resulting from the formation of circuitry on the working surface of the substrate, differential polishing will physically destroy any die which were formed in the over-polished areas.
Edge over-polishing is caused by several factors. Uneven distribution of the polishing enhancing slurry on the surface of the substrate is one factor which contributes to edge over-polishing. Where the slurry is more rapidly replenished, such as along the edge of the substrate, the substrate is more rapidly polished. Another factor is relative pressure between the polishing pad and the substrate at different locations on the substrate. The areas where the pressure is higher have higher polishing rates. One relatively high pressure area occurs where the substrate edge presses into the polishing pad, which causes the substrate edge to polish more rapidly than the substrate center. An additional factor, for a polishing apparatus in which the polishing pad and the substrate both rotate, is the cumulative motion between the substrate and the polishing pad. The cumulative motion may be higher near the edge of the substrate than at the substrate center. The greater the cumulative motion between the polishing pad and the substrate, the greater the quantity of material removed from the substrate. As a result of these and other factors, the substrate edge is usually polished at a higher rate than the substrate center.
Substrate over-polishing may also occur in non-contiguous areas of the substrate. This over-polishing is commonly attributed to a warped or otherwise improperly prepared substrate and is exacerbated by the mounting system which affixes the substrate to the carrier. The carrier commonly includes a generally planar lower face. A conformable material is located on this lower face to receive the substrate there against. The conformable material may be a polymer sheet, or it may be a wax mound over which the substrate is pressed to form a conformable receiving surface. The conformable material, and the lower face of the carrier, may not be as flat as the desired flatness of the substrate. Therefore, the conformable material and generally planar lower face may include protrusions which differentially load the back side of the substrate when the substrate is located on the polishing pad. This differential loading will create overloaded areas on the surface of the substrate engaged against the polishing pad which correspond to the location of the protrusions of the lower face and conformable material. In the localized areas of the substrate where this overloading occurs, the substrate will be over-polished, and the die yield from the substrate will be reduced.
In addition to the reduced die yield which results from the creation of over-polished areas on the substrate, the use of a large rotating polishing pad to sequentially process substrates is inherently inefficient. Typically, the surface area of the substrate is no more than 20% of the surface area of the polishing pad. Therefore, at any point in time, most of the polishing pad material is not in contact with the substrate. One way to increase the utilization of the surface area of the rotating polishing pad is to simultaneously process multiple substrates on the polishing pad. However, users of CMP equipment are reluctant to do so because a substrate may crack or may otherwise be defective, and chips or other contaminants will be transferred by the rotating polishing pad to all of the substrates being simultaneously processed on the polishing pad.
Therefore, there exists a need for a CMP polishing apparatus which provides (i) greater uniformity in the material removal rate between each discrete location or region on the face of the substrate and (ii) greater polishing pad utilization.
The present invention is a chemical mechanical polishing apparatus and method which includes multiple embodiments useful for increasing the uniformity of the material removal rate, or the utilization of a polishing pad, of chemical mechanical polishing equipment. In a first embodiment, the apparatus includes a substrate carrier which differentially loads selected portions of the outer surface of the substrate against the polishing pad. Where edge over-polishing occurs, the carrier may be configured to increase the pressure between the polishing pad and substrate at the center of the substrate to compensate for a high material removal rate which would otherwise occur adjacent the edge of the substrate.
In a second embodiment of the invention, the carrier is configured to load all portions of the outermost surface of the substrate equally against the polishing pad. By equally loading the substrate against the polishing pad, the incidence of localized over-polishing caused by protrusions on the conformable material or the carrier lower surface may be reduced or eliminated. To further control edge over-polishing which occurs as a result of greater cumulative movement between the substrate and the polishing pad at the substrate edge, the substrate may be orbited on the polishing pad while the polishing pad is slowly rotated. The carrier may be controlled to orbit the substrate without rotation or to rotate the substrate at a desired velocity as it is orbited. By closely controlling the rotational velocity of the substrate in comparison to the rotational velocity of the polishing pad, the mount of differential polishing of the substrate caused by differential cumulative movement at different discrete locations or regions of the substrate may be reduced or eliminated.
In a third embodiment of the invention, multiple substrate carriers are provided for simultaneously loading multiple substrates on a single polishing pad. In one sub-embodiment of the multiple carrier embodiment, the polishing pad is rotationally oscillated. By rotationally oscillating the polishing pad, the area of the polishing pad which contacts any one of the multiple substrates may be isolated from the area of the polishing pad contacting any other substrate. In an additional sub-embodiment of the invention, the polishing pad includes a groove or grooves therein, which are configured to collect any chipped portion of a substrate which may be created during processing. In a further sub-embodiment of the multiple carrier embodiment of the invention, the polishing pad is maintained in a stationary position, and a multi-lobed groove is located in the polishing pad immediately below the location at which the substrate is received on the polishing pad. The multi-lobed groove provides areas of contact and non-contact between the substrate and the polishing pad, and the slurry may be replenished in the areas of non-contact between the substrate and the polishing pad.